Engines may operate using a plurality of different fuels, which may be separately delivered, or delivered in varying ratios, depending on operating conditions. The different fuels may result in differing engine efficiencies at a given operating condition. For example, an engine may use a first fuel (e.g., ethanol) and a second fuel (e.g., gasoline), each with different knock suppression abilities, to reduce engine knock limitations while improving overall fuel economy. As such, there may be several reasons why different fuels available to the engine may different efficiencies at various engine running conditions. As one example, the available fuels may have different octane ratings which affects spark retard usage and engine efficiency at high loads (for example, when the different fuels are compressed natural gas versus gasoline, or E85 versus gasoline, or regular grade fuel versus premium grade fuel). As another example, different fuels may result in different engine pumping work (for example, when the different fuels include a gaseous fuel versus a liquid fuel, or a port injected fuel versus a direct injected fuel). As still another example, different fuels may result in different parasitic losses (such as when the fuels include a fuel delivered via high pressure direct injection versus a fuel delivered via low pressure port injection).
Engine control systems may select a fuel for injecting into cylinders from the multiple available fuels based on engine operating conditions, fuel availability, as well as fuel costs. One example approach is shown by Surnilla et al. in U.S. Pat. No. 7,703,435. Therein, fuel selection is based on fuel availability, engine temperature, and knock limits. Another example approach is shown by Williams et al. in US20140067540. Therein fuel selection is based on fuel costs in a geographical area of interest.
However the inventors herein have recognized potential issues with such approaches. As one example, the optimal fuel economy gain associated with adjusting fuel usage may not be realized due to the fixed gear ratio of the transmission. In particular, at a given driver demand, for a fuel selected for use in the engine, there may be an associated fixed engine speed and load range that meets the driver demand. An engine controller may transition to a more efficient or cost-effective fuel for the driver demand. However, upon changing fuels, there may be engine limitations experienced at the associated engine speed-load that may reduce the fuel economy benefit of the fuel transition. As an example, upon transitioning to a lower octane fuel, the engine may become more knock-limited at high loads. The fuel penalty associated with the knock mitigation may outweigh the fuel economy benefit of the fuel transition. As another example, upon transitioning to a fuel delivered via high-pressure direct injection, the engine may become more friction limited at low loads compared to a fuel delivered via low-pressure port injection. Another issue is that frequent changes in operator pedal demand may cause the engine load to move back and forth, leading to frequent switching between fuels. Excessive switches between fuels can degrade fuel economy due to losses incurred during transitions.
The inventors herein have recognized that the fuel economy benefits of a multi-fuel engine may be better leveraged through integration with a continuously variable transmission (CVT). In particular, the CVT may enable the engine speed and load to be adjusted while maintaining the more cost-effective and efficient fuel and while maintaining the power output of the engine. In one example, fuel economy may be improved by a method for a multi-fuel engine coupled to a CVT comprising, for a power level, comparing operating cost at a current fuel to operating cost at an alternate fuel with an adjusted engine speed-load; and in response to a higher than threshold improvement in the operating cost at the alternate fuel with the adjusted engine speed-load, transitioning to the alternate fuel and changing to the adjusted engine speed-load. In this way, an engine can be operated with a fuel that provides an improved fuel economy for a given driver demand without being excessively knock limited at higher loads. In addition, the need for frequent fuel switching can be reduced.
As one example, an engine may be configured as a bi-fuel engine that uses one of two fuels for propelling vehicle wheels via engine torque. The two fuels may have different octane ratings and may be delivered to the engine via distinct delivery systems. As one example, the two fuels may include a higher octane ethanol fuel that is delivered to an engine cylinder via direct injection and a lower octane gasoline fuel that is delivered to the engine cylinder via port injection. At any given driver demand, the controller may be configured to compare the fuel efficiency versus power for each available fuel, including a fuel the engine is currently operating on as well as an alternate available fuel. The effects of fuel octane and associated knock limits are included in the efficiency versus power information. The effects of parasitic losses (such as high pressure direct injection) are also included in the efficiency versus power information. Upon retrieving a cost of each fuel (such as via wireless communication with a server, or from the cloud), the efficiency may be divided by the cost to determine a “work per dollar” value for each fuel. Then, if the more cost efficient fuel (that is, the one that provides more work per dollar spent on fuel) is not the current fuel, the controller may predict if there are any limitations, such as knock limitations, associated with the corresponding engine speed-load. If so, the controller may further determine if the engine speed-load can be changed while maintaining usage of the cost efficient fuel and while maintaining the demanded engine power output, and any fuel penalties associated therewith. In other words, the controller may determine whether the optimum engine speed-load with the more cost efficient fuel is different from the current engine speed-load. If the engine speed-load can be changed while maintaining usage of the selected fuel with a net fuel economy improvement, the controller may proceed to operate with the selected fuel and shift to the optimum speed-load range for the selected fuel. Else, the engine may switch to operating with the other available fuel. As an example, upon transitioning to a lower octane fuel, for a given driver demand, the engine speed may increase while the engine load decreases. To address knock anticipated while using the lower octane fuel, an engine controller may actuate the CVT to increase the engine speed while decreasing the engine load so as to maintain the demanded engine power output while providing a net cost benefit. Likewise, when transitioning to a higher octane fuel, the engine speed may be lowered (from the previous engine speed for the lower octane fuel) while load is increased (as compared to the previous load for the lower octane fuel).
In this way, fuel economy benefits can be improved. The technical effect of integrating multi-fuel engine technology in a vehicle having a CVT transmission is that for a given driver demanded power, the benefits of the various fuels can be better leveraged. In particular, the engine speed and torque for a given driver demanded power can be adjusted to reduce knock limitations at higher loads and friction losses at lower loads, while accounting for changes in fuel properties. The technical effect of assessing the fuel economy benefit of switching fuels with the fuel penalty associated with operating at the engine speed-load profile corresponding to a selected fuel is that frequent fuel switching can be reduced. While operating the engine with the more efficient and cost-effective fuel, CVT adjustments can be used to extend engine operation with the more efficient and cost-effective fuel despite changes in driver or wheel torque demand.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.